1904.] Properties of the Alloys of Silver and Cadmium. 227 



the more uniform the composition should be. It should be possible 

 to prepare perfectly uniform alloys of any composition containing more 

 than 80 per cent, of silver by re-heating them to about 850 for a 

 number of hours. A practical difficulty in carrying this out would be 

 that the cadmium would be partly volatilised, so that it would be 

 necessary to remove the outer layers of the ingots after heating 

 them. 



On the other hand, Osmond has pointed out that it is by no means 

 uncommon for crystallites to be developed by the attack of suitable 

 re-agents in media which are chemically homogeneous. He cites the 

 bronzes and the gold-copper alloys as instances. According to this 

 view the specimens showing crystallites are already uniform in com- 

 position and do not require to be annealed in order to become so. It 

 is to be observed, however, that the gold-copper alloys have been 

 shown to be non-homogeneous.* Osmond does not state to what 

 bronzes he refers, but the copper-tin alloys are now well known to be 

 heterogeneous also. It may be repeated that in practice the 92 '5-per- 

 cent, alloy is found, on casting, to be uniform in composition, so that 

 it is unnecessary to anneal it. 



In the alloys containing from 80 70 per cent, of silver, the 

 crystallites in slowly cooled specimens are larger and more regularly 

 cross- or fern-shaped than in those with more silver (see fig. 4, 

 Plate 10). The matrix is readily dissolved out by nitric acid, leaving 

 the crystallites in relief surrounded by a deep-sunk network. The area 

 occupied by the crystallites is reduced, and that occupied by the 

 matrix increased as the percentage of silver falls. The equilibrium 

 of the systems at lower temperatures has not been examined in this 

 part of the series. 



The alloy corresponding to the formula Ag 2 Cd is much finer grained 

 than either the 64- or the 70-per-cent. alloy. Etching with nitric acid 

 leaves an excessively fine network in relief with darker pittings 

 between, but no regular structure is observable. Other reagents are 

 equally ineffective in developing the structure. Ee-heating at 750 for 

 1J hours, and subsequent chilling, merely increases the size of the 

 network without in any other way changing its appearance. One 

 curious effect of heating to 750 these specimens embedded in sand is 

 that the cadmium on the surface is volatilised, leaving a layer of pure 

 silver. On removing this during the operation of polishing, a black 

 layer is encountered coloured by oxide of cadmium, and underneath 

 this the original alloy is found to exist. The layers are not everywhere 

 of the same thickness, so that in the course of polishing alternate rings 

 of white and black are produced, resembling the well-known Japanese 

 decorative metal-work known as Mokume, which is used in jewellery. 



The cooling curves indicate that the compound Ag 3 Cd 2 separates 



* Roberts- Austen and Rose, 'Roy. Soc. Proc.,' vol. 67 (1900), p. 105. 



